The present invention relates, in general, to a micromachined sensor, and more particularly to a double pinned micromachined sensor utilizing a composite film having a net tensile stress.
A wide variety of micromachined sensors utilize a plate suspended over a silicon substrate. This type of micromachined sensor includes accelerometers, pressure sensors, and the like. The spacing between the plate and the silicon substrate is made to change by the force being measured. This variation in spacing typically produces a proportional variation in capacitance value which is used to detect movement of the plate relative to the silicon substrate. Surface micromachined capacitors of this kind require a plate which remains parallel to the surface so that the quiescent capacitance value may be controlled. A similar form of sensor uses a closed loop control system in which a differential capacitor is used to sense changes in the position of the plate and to apply a compensating electrostatic force to minimize movement of the plate. A sensor which uses a closed loop control system is more tolerant of the spring constant, but requires precise centering of the movable plate between upper and lower fixed plates. In addition, the active positioning system typically must rely on the springs and geometric design of the sensor to control parallelism. A plate which remains parallel to the surface as a result of the spring geometry is thus even more desirable with the closed loop control system.
In the past, sensors have typically been structures supported only by one end, that is a "singly pinned" beam. Parallelism of these singly pinned structures has been controlled by balancing the vertical gradient of the polysilicon film. Compressive stresses in the top and bottom of the polysilicon film are balanced by adjustment of deposition, annealing and doping conditions. These adjustments are difficult to control well enough to equalize stresses so as to give both an adequate balance and acceptable strength This problem limits the maximum dimension of the dynamic element and complicates manufacturing. A double pinned structure supported by multiple supports which are under tension is inherently more parallel to the surface. A double pinned structure of this kind requires a tensile film, but heavily doped polysilicon films are characteristically compressive. As a result a polysilicon film which is heavily doped to provide electrical conductivity cannot be used to fabricate such a double pinned structure.
A double pinned structure utilizing beams made from a polysilicon film is described in U.S. Pat. No. 5,090,254 entitled "Polysilicon resonating beam transducers", issued on Feb. 25, 1992, to H. Guckel et al, assigned to Wisconsin Alumni Research Foundation, and which is incorporated herein by reference. This patent describes a polysilicon film which is typically deposited at a rate of 68 angstroms per minute and then annealed to produce a zero or low tensile stress beam. The rate of deposition is significantly slower than the 120 angstroms per minute typically used in semiconductor manufacturing. When building a film that is typically 20,000 angstroms thick the slow rate of deposition adds several hours to this deposition step alone. The annealing required is incompatible with the requirements of some semiconductor manufacturing. Furthermore, this method uses substantially undoped polysilicon which requires that a separate doping step be performed. This method cannot produce a structure having a well controlled film stress or a high tensile stress. For example, the method disclosed does not allow enough control over film stress to form selected regions of tension and rigidity in the film.
There is a need for a film suited for fabrication of sensors which exhibits a tensile stress. The film should be compatible with standard semiconductor manufacturing methods, use heavily doped polysilicon, and be formed into desired patterns by commonly used photolithographic methods.